Networks are formed in data centers by using a plurality of servers and a plurality of switches. One or more virtual machines (VM) may operate in each of the plurality of servers. Communication between the plurality of virtual machines is relayed through the switches. A management server is included among the plurality of servers. The management server maintains topology information for the entire network and manages the communication paths in the network. The switches send information indicating their own states to the management server in predetermined cycles. The servers send information (flow information) pertaining to flows that originate from virtual machines operating therein, to the management server. The management server uses the information obtained from the switches to detect the occurrence of congestion in the network. Then, the management server uses the flow information obtained from the servers to reduce the congestion by performing a process to change the transfer path of the flow passing through the switch in which the congestion has occurred.
Japanese Patent Laid-open Publication No. 2013-61840 describes a system that includes an apparatus management device and a fault management device that analyzes the history of faults that have occurred in the system and predicts a fault that may occur in an electronic apparatus after a predetermined period of time has passed. The apparatus management device shortens an interval for obtaining information from the electronic device in which the occurrence of a fault is predicted. Japanese Patent Laid-open Publication No. 2005-242564 describes a host machine that, upon notification of an abnormal state from a printer, requests the printer having the abnormal state to transmit periodic operation information notifications at a shorter communication interval than an initial communication interval up to a prescribed time. The host machine requests printers not having an abnormal state to send operating information notifications at intervals longer than the initial communication intervals up to a prescribed time. Japanese Patent Laid-open Publication No. 2011-166466 discusses a switch that transmits a loop detecting packet having a media access control (MAC) address of the switch registered therein, and that notifies a management device when a loop detecting packet that includes the MAC address of the switch is received. Japanese Patent Laid-open Publication No. 2006-164038 discusses an analyzing device that detects transmission of packets to the same host, whose number is a prescribed value or greater, during a fixed time period, and then determines that the transmission is an attack flow.